Near-infrared radiation can generally pass through layers of skin and fat to illuminate blood vessels in muscle tissues. The radiation can be absorbed by hemoglobin in red blood cells, myoglobin in muscle fibers, water, and other proteins in blood plasma. Radiation is scattered by both muscle fibers and blood cells, and the scattered radiation can be detected and analyzed to determine the wavelength dependence of the scattered radiation. The absorbance spectrum of the various absorbing components in muscle tissues can be determined by comparing the spectra of incident radiation delivered to the tissues and the scattered radiation from the tissues. In some cases, particular spectral features in the absorbance spectrum can be assigned to particular components in the muscle tissues (e.g., certain spectral signatures can be assigned to absorption by hemoglobin and/or myoglobin).
During cardiac events such as ventricular fibrillation, pH and muscle oxygen saturation in tissues change, and these changes in pH and muscle oxygen saturation can reflect the physiological state of the subject, and may even have predictive value. In a typical cardiac arrest emergency situation, emergency medical assistance must be administered in a finite amount of time, and the decision as to what type of therapy to administer can be critical for the subject's survival. In many cases, there are no available witnesses to provide information about the cause of and circumstances surrounding the subject's condition. Given the narrow window of time in which emergency treatment must be administered, as well as the lack of information concerning the subject's condition, there is a need for a fast and accurate method of estimating the onset of the cardiac arrest emergency and evaluating the effectiveness of the emergency treatment being administered.